You can use the html snippet to load the html file or just copy and paste the html into this widget.
If you want to use an existing html file then you could place this in the theme directory and then open the file with a link button making sure the target is your environment url followed by /htmlfile.html
If you are running on localhost:8080 and the file is named testMx.html the link button target should be: http://localhost:8080/testMx.html
I did try it this way.
I put a 'test.html' under: <myproject folder>/theme
And I put http://localhost:8080/test.html
But I got 404 error shown below. Is there security related to this?